We already know that sapphire is much more expensive to use right now than the Corning Gorilla Glass that Apple has traditionally used for iPhone displays but it also has to be in virtually flawless condition before being used or else it becomes quite brittle, Technology Review says.

“Any flaws or scratches dramatically reduce the strength,” Neil Alford, a material engineering professor at Imperial College of London, explains to Technology Review.

Robert Ritchie, a professor at the University of California, Berkeley, similarly tells Technology Review that any defects in a pane of sapphire glass make it much more prone to scratches, which in turn make it much more prone to serious cracks. Ritchie also says that you need to design a smartphone just right so that the sapphire display doesn’t take the brunt of the impact during drop tests.